Add support for explicit document start marker --- in YAML docs

[guillp]

This adds an option --explicit-start which toggles the corresponding option in ruamel.yaml when formatting single-doc files.
Single doc files will then contain an explicit document start marker ---.
This doesn't change anything for multiple-docs files, which are already formatted with --- markers at the beginning and between each doc.

Enabling this option allows this pre-commit hook to produce outputs that match the default yamllint config which warns when the document-start marker is missing.